The goal of this project is to characterize PAF acetylhydrolase (AH), the enzyme that specifically hydrolyzes and inactivates the potent inflammatory mediator platelet-activating factor (PAF). PAF is a phospholipid hormone that activates platelets, neutrophils and other inflammatory cells via a specific receptor resulting in marked hemodynamic effects. PAF infusion causes anaphylaxis, shock and death. Antagonists that block the PAF receptor prevent these effects and also are efficacious in models of ranging from hemorrhagic or septic shock to cardiac and mesenteric ischemia/reperfusion damage. The potent and myriad effects of PAF indicate that its presence will be highly controlled. However, we have recently shown that uncontrolled oxidation of certain membrane phospholipids, or those in low density lipoproteins, results in the formation of compounds that sufficiently mimic PAF that they can activate the PAF receptor. Thus, control of PAF synthesis may not be sufficient to regulates its presence. The enzyme to be examined in this project specifically inactivates PAF, and the mimetics derived from oxidative-fragmentation of phospholipids, by hydrolyzing their sn-2 residues. These residues must be short to be recognized by either the PAF receptor or AH, so this enzyme can circulate in a fully active state without attacking intact phospholipids. This enzyme is found in blood associated with low and a specific class of high density lipoproteins (LDL and HDL, respectively), and can transfer between these with high specificity. The catalytic efficiency of AH is strongly affected by its environment, and when associated with HDL it is barely active. This is important as our current data show that AH can protect LDL from oxidative modification to an atherogenic particle postulated to be an initiating event in atherosclerosis. We postulate that AH attacks oxidized phospholipids and removes the reactive group before it can derivatize apoB to its modified form. We also postulate that HDL donates AH to LDL and thereby distributes the limited amount of All among LDL particles. HDL (and in fact only those subfractions that contain AH) has been shown to protect LDL from modification, so we postulate that AH plays a protective role in atherosclerosis in addition to a protective role in pathologic inflammation. The Specific Aims of the proposed research are: 1) Complete the cloning of the plasma AH 2) Define the basis for AH association with specific lipoprotein classes. 3) Determine how AH protects LDL from oxidative modification. 4) Determine if AH will protect in models of shock; ischemia/reperfusion.